Contemporary pyroelectric detectors generally function by using heat to generate an electric voltage in a material. The material is deemed pyroelectric when a change in temperature alters its spontaneous polarization, causing a change in electric voltage across the material to develop. An optical absorber proximate the pyroelectric material converts incident radiation into heat; the absorption spectrum of the optical absorber generally determines the wavelength response the detector and generally covers at least 1000 nm of optical bandwidth. When the electric voltage across the material is directly measured, it is generally called a “voltage-mode” pyroelectric detector. When the current driven in the detector by the change in voltage is measured, it is generally called a “current-mode” pyroelectric detector. Current-mode pyroelectric detection is considered to be superior to voltage-mode because it minimizes electromagnetic interference and stray detector capacitance. See, M. Chirtoc, E. H. Bentefour, J. S. Antoniow, C. Glorieux, J. Thoen, S. Delenclos, A. H. Sahraoui, S. Longuemart, C. Kolinsky and J. M. Buisine, “Current Mode Versus Voltage Mode Measurement of Signals from Pyroelectric Sensors,” Rev. Sci. Inst. 74, 648-650 (2003) (“Cirtoc, et al.”).
In one instantiation of the current art shown in D. Dao, S. Ishii, T. Yokoyama, T. Sawada, R. P. Sugavaneshwar, K. Chen, Y. Wada, T. Nabatame and T. Nagao, “Hole Array Perfect Absorbers for Spectrally Selective Midwavelength Infrared Pyroelectric Detectors,” ACS Photonics 3, 1271-1278 (2016)(Ishii, et al.), a plasmonic perfect absorber was incorporated with a pyroelectric material to fabricate a detector that selectively detects wavelengths of λ=3.88 μm and 5.50 μm. A plasmonic perfect absorber is generally a sub-wavelength patterned structure designed to match the impedance of radiation in air to the impedance of the radiation in the patterned structure. The condition of matched impedances causes minimal reflection and maximum absorption in the structure. Incident radiation with wavelengths that do not satisfy the impedance matching condition are generally reflected. Ishii, et al. states that the detector “can be used for various applications such as temperature sensing, IR color imaging, NDIR [Non-Dispersive InfraRed] spectroscopy, and IR material sensors.” This detector is operated in voltage mode.
In the referenced instantiation, an array of 1.8 μm diameter apertures with a period of 3.0 μm were fabricated in gold using colloidal mask lithography. Underneath the array of holes in gold was an oriented zinc oxide (ZnO) layer 680 nm thick. Underneath the ZnO layer was a platinum (Pt) electrode and a silicon (Si) substrate. The ZnO layer was grown in such a way that the measure of crystallinity (X-Ray diffraction full width at half maximum) was 1.37° (see, Mirica, E., G. Kowach, P. Evans, and H. Du, “Morphological Evolution of ZnO Thin Films Deposited by Reactive Sputtering,” Cryst. Growth and Design 4, 147-156 (2004)). Crystallinity is measured using x-ray diffraction; the closer the measure of crystallinity is to 0.00° , the more crystalline (“highly oriented”) a material becomes.
Pyroelectric detectors are commonly used to detect and characterize laser radiation, such as the Pyrocam IV manufactured by Ophir Optronics. These detectors do not selectively detect one laser wavelength and generally have very low laser induced damage thresholds. In the specific case of the Pyrocam IV, it is sensitive from the X-Ray region to the THz region. Detectors such as photoconductors as taught by U.S. Pat. No. 7,683,310 to Sinclair, et al. and U.S. Patent Application Publication No. 2008/0002192 by David, are operable to detect both scattered and direct laser radiation and can be cooled or uncooled depending on the required detector sensitivity.
There are several drawbacks to the current art. While the pyroelectric ZnO as disclosed by Ishii, et al. shows a degree of crystalline orientation, processes of reactive sputtering, such as utilized to produce ZnO, are capable of producing films having a significantly higher crystalline orientation, and the degree of crystalline orientation has a direct effect on the sensitivity of a pyroelectric detector. The higher the crystallinity of the pyroelectric material (i.e., the more highly oriented pyroelectric material), the more sensitive the detector. Commercially available laser detectors generally cannot survive irradiation from excessively high energy sources and therefore require external protection such as sacrificial optical limiters to protect them from damage. Furthermore, they generally lack the ability to selectively detect individual laser lines without external filter wheels.
Accordingly, there is a need in the art for a detector capable of detecting specific laser wavelengths that does not require external protective devices or external filter wheels.